Studies of the transmural distribution of myocardial blood flow will be performed in chronically instrumented awake dogs using 15 micron diameter radionuclide labeled microspheres injected into the left atrium. A study will be carried out in dogs trained to run on a motor driven treadmill to examine the relationship between regional myocardial blo d flow and coronary venous metabolites during exercise in the presence of the coronary artery stenosis. Exercise will be carried out in the presence of several degrees of coronary artery stenosis while coronary sinus blood is obtained for determination of adenosine, inosine, hypoxanthine and lactate. In a second study, the effects of beta-2 adrenergic blockade and stimulation on the intercoronary collateral blood flow following acute coronary artery occlusion will be examined. This study will determine whether beta-2 receptor stimulation results in vasodilation of the coronary collateral vasculature with increased blood flow to areas of ischemic myocardium. A study will be carried out to examine the myocardial perfusion boundary which separates ischemic from normal myocardium following acute coronary artery occlusion in the dog. In addition, several studies will evaluate the ability of the coronary vasculature to direct blood flow toward areas of greatest ischemia by examining the transmural distribution of perfusion during reactive hyperemia following total and subtotal coronary artery occlusions. Finally, studies will be carried out to examine the relationship between myocardial blood flow and tissue uptake of the antiarrhythmic drug, procainamide, following acute coronary artery occlusion in the dog.